Earlier this week, I kept getting WSOD on a Drupal site. There was no hint in syslog or the Apache error log. The access log, however, had a lot of info and using cut made it a lot more readable. Finally, the reference to httprl_async_function_callback caught my eye. We were using advagg with httprl. It turned out that if httprl is enabled and advagg is not able to make a self-request, it will result in WSOD.

So here it is for reference:

sudo tail /var/log/httpd/ssl_access_log | cut -c -150
10.0.0.1 - - [26/Sep/2016:17:30:42 -0400] "POST /httprl_async_function_callback?count=1 HTTP/1.0" 404 228
10.0.0.1 - - [26/Sep/2016:17:30:50 -0400] "POST /httprl_async_function_callback?count=2 HTTP/1.0" 404 228
10.0.0.1 - - [26/Sep/2016:17:30:56 -0400] "POST /httprl_async_function_callback?count=3 HTTP/1.0" 404 228
10.0.0.1 - - [26/Sep/2016:17:31:04 -0400] "POST /httprl_async_function_callback?count=0 HTTP/1.0" 404 228